Researchers in Switzerland have documented a rare case of an HIV-positive patient contracting a second, genetically different strain of the virus two years after being initially infected. Bernard Hirschel of the University of Geneva wrote in the September 5 edition of The New England Journal of Medicine that his team was able to document the case because the patient is enrolled in an ongoing AIDS drug study to test early treatment of the virus. After two years of successfully controlling viral replication through medication and an experimental vaccine designed to boost immune system responses to his infection, the 38-year-old man suddenly began experiencing rising viral levels. The researchers determined that he had acquired a second HIV infection through unprotected sex with anonymous sex partners while on vacation in Brazil and that the new viral strain was able to thwart the protections offered by the experimental vaccine.

A similar case study of a Boston man enrolled in an HIV vaccine study who became infected with a genetically different strain of HIV through unprotected sex was presented in July at the XIV International AIDS Conference in Barcelona. Researchers in Thailand also have documented two cases of HIV superinfection that occurred through intravenous drug use.

Many doctors had assumed that a patient's natural immunity would keep them from getting HIV more than once, but the newly confirmed cases of second HIV infections support warnings by some AIDS experts that superinfection is possible. The findings also suggest that HIV-positive people need to continue to practice safer sex, even with other infected sex partners, the researchers concluded. "With sexual activity seemingly increasing among persons with HIV-1 infection, this is a public health message that needs to be broadcast loud and clear," the researchers wrote.

The findings also may foreshadow problems in AIDS vaccine research, particularly studies of vaccine candidates that aim to stimulate the immune system to better control HIV in the body. A vaccine candidate that works well in controlling a patient's HIV may be ineffective against a second, genetically different version of the virus, which is what happened with both the Swiss and Boston patients. Because of this, it may not be possible for a single vaccine to be effective against many different strains of the virus. It was also noted that genetic variations in HIV are greater than those in the flu virus, which requires a new vaccine every year.